1. Field of the Invention
This invention relates to static inverters and more particularly to the class of static inverters known as switching mode regulators. In a switching mode regulator, regulation is achieved by control of the duty cycle of an intermittently operated switch.
2. Description of the Prior Art
In a conventional switching mode regulator, a dc source is coupled through an intermittently operated transistor switch to a current maintaining inductance and a load. The switch is normally conductive for a certain percentage of the time and nonconductive for a certain percentage of time and in the usual case the inductance is made sufficiently large in relation to the load to sustain load current while the switch is nonconductive. In order to provide a suitable current path for inductively sustained current, a flyback diode is provided. The transistor switch may take several forms, but one convenient form, which is not in itself a part of the present invention, is one in which the transistor switch is turned on at a controlled rate by a trigger oscillator and turns itself off by an internal feedback mechanism.
During normal operation of a switching mode regulator, transitions occur where the switching transient turns on before the "catch" or flyback diode has been completely turned off. The delayed turn off is attributable to stored charge in the catch rectifier, and unless protective measures are adopted, the transistor switch may be forced to supply a very large uncontrolled current until stored charge has been removed from the diode. The large current over stresses the transistor. Accordingly, to achieve reliability, either a large or a more costly transistor must be used or some protective measure to control the transistor must be provided. In addition to the stress which occurs when the transistor switch is first turned on, the transistor switches are also subjected to very large and steep voltage changes produced by the release of energy stored in the inductor, when the transistor turns off. If the collector current falls relatively slowly while the voltage across the transistor (Vce) rapidly increases to the full value of the supply potential, the transistor will be subject to both additional loss and the possibility of reverse secondary breakdown. While limiting the voltage change (dv/dt) by snubbing action has been proposed, conventional "snubbers" are not only expensive but lossy.